Present day cellular mobile telephone systems provide for a large and increasing demand for mobile services. Cellular systems “reuse” frequency within a group of cells to provide wireless two-way radio frequency (RF) communication to large numbers of users.
Each cell may cover a small geographic area and collectively a group of adjacent cells may cover a large geographic region. Each cell uses a fraction of the total amount of RF spectrum available to support users (cellular users) at mobile stations, such as a mobile phone handset or a personal computer. Cells may be of different sizes (for example, macro-cell or micro-cell) and are generally fixed in capacity. The actual shapes and sizes of cells are complex functions based on terrain, the man-made environment, the quality of communication and the user capacity.
Cells are typically connected to each other via communication links, such as telephone lines or microwave links. The cells, in turn, may be connected to a packet-switched network, such as a Voice Over Internet Protocol (VoIP) network, through switches that are adapted to interface the cells with the packet-switched network. These switches typically provide various functions to support communication with cellular users, such as the hand-off from cell to cell as a user moves between cells.
In conventional cellular systems, each cell has a base station with RF transmitters and RF receivers co-sited for transmitting and receiving communications to and from cellular users in the cell. The base station employs forward RF frequency bands (carriers) to transmit forward channel communications to users and employs reverse RF carriers to receive reverse channel communications from users in the cell.
The forward and reverse channel communications use separate frequency bands so that simultaneous transmissions in both directions are possible. This operation is referred to as frequency division duplex (FDD) signaling. In time division duplex (TDD) signaling, the forward and reverse channels take turns using the same frequency band.
In addition to providing RF connectivity to a user, a base station typically provides connectivity to a mobile switching center (MSC). Often, one or more MSCs are used to cover a particular geographical region. Each MSC may service a number of base stations and associated cells in the cellular system and typically supports switching operations for routing calls between other systems (such as the PSTN) and the cellular system or for routing calls within the cellular system.
A base station subsystem (BSS) typically comprises a base transceiver station (BTS) and a base station controller (BSC). A BTS is typically controlled by the MSC via a base station controller (BSC) associated with the BTS. The BSC assigns RF carriers to support calls, coordinates the handoff of mobile users between base stations, and monitors and reports on the status of base stations in the BSS. The number of base stations controlled by a single MSC may depend upon the amount of traffic at each base station, the cost of interconnection between the MSC and the base stations, the topology of the service area and other similar factors.
The BTS includes radio transceivers that define a radio cell boundary and handles various radio interface protocols (e.g., Urn) with the mobile station. Moreover, the BTS typically supports all the required channel coding, encryption and speech coding required by the radio interface. The speech transcoding may be performed locally at the BTS or remotely at the BSC or MSC.
The BSC manages the radio resources of one or more BTSs. The BSC controls most of the features of the radio network, including allocation of radio time slots to a mobile station, release of the resources, interpretation of measurement results and control of radio interface handovers. The BSC interfaces to the BSS with the MSC.
The MSC provides functions required for switching calls to/from a mobile station and the packet-switched network or a mobile station to another mobile station. In addition, the MSC also provides various functions needed to track and maintain communication with the mobile station, such as call progress tone generation, registration, authentication, location updating and inter-MSC handovers.
Call progress tones are audible tones sent to calling parties to indicate the status of calls. Examples of call progress tones include a dial tone, ringing tone, busy tone, warning tone and so on. In addition, other types of tones, such as signaling tones, which may be in-band or out-of-band are used in a wireless network to provide support functions and services, such as call handover from one BSS to another, radio channel assignment, data services, emergency provisioning and the like.
The Internet and other packet-switched networks are increasingly used to replace analog TDM networks as a transmission medium for voice telephone calls. Voice over Internet Protocol (VoIP) telephony software and services now provide low cost, or even free, telephone calls anywhere in the world. With simple equipment at the subscriber end, a virtual connection can be established between two callers through a system of interconnected packet-based networks that may include the Internet, intranets or other digital networks. VoIP is thus emerging as a viable alternative to legacy trunked networks, as long as occasional delays and sometimes inferior quality of service can be tolerated.
Traditional Time Division Multiplexing (TDM) or similar trunked networks have been designed to carry signaling tones used in a wireless network. However, emerging network architectures have replaced these traditional networks with packet-switched networks, such as VoIP networks and other similar packet-switched networks that use packetization and voice signal compression, which typically do not support carrying signaling tones generated by, e.g., wireless networks. Consequently, to effectuate the use of these packet-switched networks with e.g., wireless networks, these packet-switched networks need to accommodate wireless network tones as well as different types of tones provided by different service providers.